During deposit of load by means of a floating crane to a stationary installation, during wave motion there may easily arise large relative movements of load relative to said installation.
Said relative movements may also produce over-loading of the construction which performs the deposit of the load.
Upon deposit of a large load, e.g. in the amount of several hundred tons, the forces which are released upon impact created by such movements may produce great damage both to the load itself and the installation. In order to decrease the risk of such impact one is presently and substantially restricted to deposit of the load when the conditions for minimum relative movements are present, i.e. during low wave motion. Thus, such operations are likely to be postponed until the wave motion conditions are satisfactory.
From the prior art, known are systems to solve problems relating to unwanted movements. In U.S. Pat. No. 3,314,657 a passive system is described having the purpose of maintaining a predetermined tension in a cable by means of hydro pneumatic means. A piston cylinder is positioned between two pulleys and by variation of the movements of the piston relative to the cylinder the cable tensioning is altered as required. The patent, however, does not teach how the task may be solved when extremely large loads as well as transients result from relative movements. A wave motion compensating system is known to maintain a load which is suspended on a floating platform at substantially constant level and comprising a passive load carrying system having a resilient load carrying coupling which can be mounted between a fixed support on the platform and a load which is to be carried. The installation is based on a closed system, where increase in liquid pressure in a wave motion compensating cylinder causes transfer of liquid from said cylinder to a shock absorbing cylinder and upon the operation of the latter an increase in pressure in an associated closed pneumatic system. The installation thus attempts to achieve a load balance by means of a slightly increased liquid pressure. Also known is a wave motion compensator which intends to maintain the loading or the position constant for an object which is suspended from a floating vessel when the vessel moves up and down on the water surface. The compensator comprises a hydraulic servo system which can offer active assistance to a passive pneumatic system, such that said loading or position is kept within predetermined narrow limits even when the movements of the vessel are quite large. The compensator, however, requires that the pressure which is present in the compensator cylinder is sufficient both to hold the load, in the example shown a drilling wire, and compensate for the present movements due to wave motion. As a consequence the "passive" compensating pressure necessarily becomes particularly high even though the "active" compensating pressure variations are small.